1. Field of the Invention
The present invention relates to a method for texturing a metallic thin film with an abrasive article, wherein the abrasive article comprises an erodible abrasive coating attached to a foraminous fibrous backing.
2. Description of the Related Art
Personal computers and their usage have become pervasive in contemporary times. Many personal computers contain a rigid memory disk or hard drive. A hard drive involves a rigid thin film metal-coated disk or nonmetal disk as the substrate of the magnetic medium. In one conventional arrangement, the thin film rigid disks are manufactured by electroless nickel plating a thin-film of nickel or nickel alloy onto an aluminum base, such as forming nickel/phosphorus (Ni-P) coating on the aluminum base. The Ni-P coating is then polished to a very fine, mirror-like finish. After polishing, the Ni-P coating is textured, followed by the application of a magnetic coating(s) thereon to form the magnetic medium.
The texturing portion of this process is critical to the ultimate performance of the rigid disks. The texturing process preferably results in a random pattern of uniform scratches with sharply defined edges in a substantially circumferential direction relative to the center of the rigid disk.
Texturing accomplishes a number of purposes. It improves the aerodynamics between the computer head (which reads and writes data on the disk) and the thin film rigid disk as the disk spins beneath the head. It also improves the magnetic properties of the coated disks. The scratches formed during texturing make it easier for the head to distinguish bytes of information between tracks on the disk. The texturing also preserves the separation between the computer head and the rigid disk when the computer is first turned on. When the computer is turned on and energized, the rigid disk will begin to spin. If the disk is smooth and untextured, this head/disk contact makes it onerous for a disk to start spinning. This phenomenon is referred to as stiction/friction in the argot of the computer industry.
To provide texturing which can deliver these and other advantages, it is imperative that the profile and topography of the texturing scratches formed in the surface of the thin film are carefully managed. For instance, if the scratches formed are too deep there may be a potential loss of data on the rigid disk. Also, it is important that the surface roughness imparted into the textured surface is relatively uniform across the width of the surface. That is, the undulations of the scratches should be as regular in pitch as possible.
Also, the portion of the metal thin film portion of the substrate abraded away during texturing is known in the industry as swarf. Practice has shown that excess swarf generated during the use of some lapping films is still apt to be present at the interface of the abrasive coating and the substrate work surfaces. Therefore, there remains some opportunity for the swarf to become attached to form high spots on the textured rigid substrate where conventional lapping films are employed. That particular phenomenon is known in the industry as reweld. Those high spots are highly undesirable as they can collide with the computer head during use, which can cause a loss of data and/or head damage as a result of the collision.
Prior to the present invention, the texturing process for thin metal films of magnetic disks was traditionally accomplished by using a loose abrasive slurry. The loose abrasive slurries provide substantially circumferential scratches that have sharply defined edges having the requisite depth. Loose abrasive slurries are, however, accompanied by a number of disadvantages. These include the inconvenience of handling the required volume of the slurry, the required agitation to prevent settling of the abrasive granules and to assure a uniform concentration of abrasive granules at the grinding interface, and the need for additional equipment to prepare, handle and also recover and recycle the abrasive slurry. Additionally, the loose slurry itself must be analyzed to assure its quality and dispersion stability requiring additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the slurry show undesirable wear. Further yet, the loose abrasive slurries are untidy; creating a large amount of debris and waste in and about the vicinity of the texturing operation. As a result, the thin film rigid disks, after texturing, must be thoroughly cleaned to remove any residues left on their surface from the abrasive slurry.
Not surprisingly, to overcome the numerous disadvantages associated with loose abrasive slurries, integral coated abrasive lapping films have been used to texture the thin film rigid disks. An example of such a product is "IMPERIAL" Lapping film (Type R3) commercially available from 3M Company, St. Paul, Minn. This lapping film comprises a polymeric film backing having an abrasive coating layer bonded thereto. The abrasive coating layer includes very fine abrasive particles (average particle size less than 10 micrometers) dispersed in a binder that is coated on the polymeric film and solidified to form a thin abrasive coating layer (about 10-15 micrometers). The surface profile of the abrasive coating is essentially flat other than the partial protrusions formed of some of the fine abrasive particles. During use, the lapping film abrades a portion of the substrate surface, thereby texturing the surface of the substrate. Similarly, U.S. Pat. No. 4,974,373 to Kawashima et al. describes an abrasive tool suited for use in lapping, polishing, texturing, and various other finishes of precision machine parts, mentioning hard disks and magnetic heads, as well as ceramics, plastics, and jewels, involving abrasive powder particles fixed in a separated proximity to each other in a binder resin coat disposed on a plastic film base to form the abrasive tool. However, in addition to problems with reweld, conventional lapping films may not provide scratches having edges as sharp and/or clean as those produced by the loose abrasive slurries. These lower quality scratch edges may degrade the quality of the disks manufactured using lapping film for the texturing process.
As a recent alternate proposal to use of such lapping films for texturing, the use of porous nonwoven cloths coated on a surface with an abrasive layer has been advanced as another method to uniformly texture thin film metal or metal alloy coated rigid disks before application of the magnetic coatings in a clean process that generates high quality scratches and avoids the problem of reweld. For example, U.S. Pat. No. 5,307,593 (Lucker et al.) discloses a nonwoven substrate coated with an abrasive layer that is used in a method for texturing magnetic media substrates having a thin-film metal or metal alloy coating, where the porous nonwoven substrate provides advantages such as the ability to collect and entrap the swarf and debris during the abrasion procedure away from the work interface, among other things. Lucker et al. employ a water-insoluble binder in the abrasive layer used to texture a thin-film metal.
U.S. Pat. No. 5,236,762 (Suzuki et al.), corresponding to European Patent Application No. 0 438 671, discloses an abrasive film suitable for use in the finishing of magnetic heads, magnetic disks, micrometers, watches, molds and so forth, comprising a film substrate having an abrasive layer on at least one side thereof, wherein the abrasive layer contains abradant particles unifromly dispersed in at least one binder selected from water-soluble macromolecular substances and water-dispersible macromolecular substances. However, the film substrates disclosed in Suzuki et al. are resinous nonfibrous films. A continuous resinous film, inherently, would have no porosity and would aggravate loading problems on the working side of the abrasive article.
What is desired in the field of rigid disk texturing is an abrasive product with the convenience of a coated abrasive product that produces results similar to that obtained with a loose abrasive slurry while avoiding the the aforementioned disadvantages associated with loose abrasive slurries.
In general, the provision of erodible abrasive films to simulate a loose slurry has been proposed. For example, U.S. Pat. No. 4,255,164 (Butzke et al.) teaches an abrasive article for use in fining ophthalmic lenses. The article comprises a flexible backing sheet and a brittle microcellular coating formed of water-insoluble modified phenol or urea formaldehyde resinous binding material, which will disintegrate during use, creating an abrasive slurry.
Also, U.S. Pat. No. 4,576,612 (Shukla et al.) teaches a polishing pad restricted to polishing glass or plastic ophthalmic lenses. The polishing pad of Shukla et al. is described as including a flexible substrate and a flexible matrix coated on said substrate involving polishing particles contained in a binder matrix composed of a latex material and a water soluble polymer. Shukla et al. refers to usage of water soluble polymers alone as the abrasive particle binder, presumably for glass polishing operations, sans the latex (viz. acrylic latex) component. However, Shukla et al. characterized the outcome for that apparent glass polishing operation as being poor.
U.S. Pat. No. 5,104,421 (Takizawa et al.) teaches a polishing pad comprising a substrate coated with a blend of abrasives and a water-soluble cellulose ether binder. Takizawa et al. also describe conventional sheet-like abrasives of sand papers and polishing tapes manufactued by bonding abrasive grains or particles on the faces of paper or fabric sheet-like substrates through synthetic water-soluble high molecular compounds such as polyvinyl alcohol or natural substances such as gelatin. However, Takizawa et al. indicate that such conventional polishing tapes manufactured by using such water-soluble bonds have no waterproofness and poor bonding strength resulting in swift falling grains from the faces of sand paper.
JP 5-228845, published 7 September 1993, teaches a polishing film for texturing of magnetic disk substrates, comprising a polymeric film and an abrasive layer comprising abrasive particles and a water soluble resin, where the abrasive particles are released from the film substrate during use in an aqueous environment.